Many olefin polymerization catalysts are known, including conventional Ziegler-Natta catalysts. While these catalysts are inexpensive, they exhibit low activity and are generally poor at incorporating α-olefin comonomers. The large variety of active sites in Ziegler-Natta catalysts makes it difficult to control polymer architecture. To improve polymer properties, single-site catalysts, in particular metallocenes are beginning to replace Ziegler-Natta catalysts.
Slurry reactors are in widespread use for production of polyethylene homo- and copolymers. Slurry reactors include stirred-tank reactors and water-jacketed tubular reactors arranged in a series of continuous horizontal or vertical loops. A “slurry solvent” in which polyethylene has low solubility constitutes the continuous phase in such reactors. The slurry is intensely stirred in a continuous stirred-tank reactor or series of reactors or, in the case of slurry loop reactors, is driven around the loop at relatively high speed by one or more rather massive pumps. Ethylene, supported catalyst, comonomers, and processing additives are injected into the reactor where polymerization takes place, creating a slurry of polyethylene in solvent.
U.S. Pat. Nos. 6,232,260 and 6,451,724 disclose the use of transition metal catalysts based upon indenoindolyl ligands. Indenoindolyl catalysts are remarkably versatile because substituent effects and bridging changes can often be exploited to provide polymers with tailored physical or mechanical properties. Unbridged indenoindolyl complexes (as exemplified in the '260 patent) usually provide favorable activity although they sometimes fail to provide polymers having high enough molecular weights. Bridged indenoindolyl complexes (as taught, e.g., in U.S. Pat. No 6,908,972) readily copolymerize α-olefins and provide polymers, with varying levels of long-chain branching. Some of the examples (e.g. Example 15 reports no long-chain branching and Mw=90,700) have very low long-chain branching (for a discussion of long-chain branching in polyethylene, see Macromolecules 39 (2006) 1474 and references cited therein).
Despite the considerable experience with single-site catalysts generally and indenoindolyl catalysts in particular, there is a need for improvement. One area that needs improvement is catalyst activity. While many complexes provide good activity, further improvement is desirable. Improvements in activity minimize the amount of complex needed. When a catalyst is selected to improve a certain polymer property, the activity is often sacrificed. Also important is the ability to incorporate α-olefins. Some catalysts that incorporate α-olefins well also produce polyethylene with high levels of long-chain branching. Other catalysts can produce polyethylene with little or no long-chain branching. Long-chain branching has a pronounced effect on rheology. For some applications, it is desirable to have a moderate amount of long-chain branching. A desirable process would easily incorporate α-olefins in order to control density and other properties. Also important is the ability to produce polyethylene with high molecular weight. There are different types of known polymerization processes and the choice of the process can influence the properties of the polyethylene. Ideally, the above-described benefits could be obtained in the commercially proven slurry process.
Thus, some processes incorporate α-olefins well; others provide polyolefins with moderate long-chain branching. Some processes can produce polyethylene with high molecular weight. Some processes have good activity. However, a slurry polymerization process having all four of these qualities is apparently not known and a compromise must be made. A valuable process would enable all four attributes.